1. Field of the Invention
The present invention relates to a flat emitter used for an electron source for an X-ray tube or for an electron source for another use, and more particularly to a technique of controlling lighting with four current-supply heating legs.
2. Description of the Related Art
An X-ray apparatus using an X-ray tube reduces a focal size of electrons upon X-raying or photographing a microscopic area, and enlarges a focal size upon X-raying or photographing a subject having a large body thickness in order to reduce a load to an anode. A structure is considered in which a plurality of emitters (also referred to as “filaments”) is prepared and each emitter is switched according to a purpose. However, preparing a plurality of emitters according to a purpose makes the structure complicated, and also increases the structure of the X-ray tube.
In view of this, a single flat emitter that can form a plurality of focal spots with different sizes has recently been proposed by the applicant of the present application (For example, see Japanese Unexamined Patent Publication No. 2012-15045). This flat emitter is referred to as a “flat double emitter” below. A structure of a conventional flat emitter including a conventional flat double emitter will be described with reference to FIGS. 7 to 15. FIG. 7 is a schematic plan view of a conventional flat emitter, FIG. 8 is a schematic plan view of a conventional flat double emitter, FIG. 9 is a diagram schematically illustrating an arrangement relation between a focusing electrode and an emitter, FIG. 10 is a schematic view of the conventional flat emitter in which the current-supply heating leg illustrated in FIG. 7 is folded at 90 degrees from its base part, FIG. 11 is a schematic view illustrating that the conventional flat emitter illustrated in FIG. 10 is assembled to a base for an X-ray tube, FIG. 12 is a schematic view illustrating a flat double emitter (double emitter of type 1) in which the current-supply heating leg illustrated in FIG. 8 is folded at 90 degrees from its base part, FIG. 13 is a schematic view illustrating that the flat double emitter (double emitter of type 1) illustrated in FIG. 12 is assembled to a base for an X-ray tube, FIG. 14 is a schematic view illustrating a flat double emitter (double emitter of type 2) in which all of current-supply heating legs are bent plural times in zigzag, and FIG. 15 is a schematic view illustrating that the flat double emitter (double emitter of type 2) as illustrated in FIG. 14 is assembled to abase for an X-ray tube.
As illustrated in FIG. 7, a conventional flat emitter has two current-supply heating legs 102 and 103 at a base part of an electron emission surface 101. An electric current is supplied from the legs 102 and 103 to heat the entire region of the electron emission surface 101 as indicated by a hatched area with positive slope in the figure, whereby thermoelectrons are emitted from the entire region of the electron emission surface 101. The thermoelectrons B (see FIG. 9) emitted from the electron emission surface 101 collide against a target T including an anode, which results in the generation of an X-ray R (see FIG. 9). The structure in FIG. 7 provides only one type of a focal spot.
In view of this, a flat double emitter illustrated in FIG. 8 is configured to form two sizes of focal spots with a single emitter. As illustrated in FIG. 8, this flat double emitter includes four current-supply heating legs 102 to 105 at the base part of an electron emission surface 101. The legs 102 and 103 out of the legs 102 to 105 are full-lighting current-supply heating legs 102 and 103 used for full lighting for a large focus in which a current is supplied to heat the entire region of the electron emission surface 101 (hereinafter referred to as “full lighting”) to emit electrons. On the other hand, the legs 104 and 105 out of the legs 102 to 105 are half-lighting current-supply heating legs 104 and 105 used for half lighting for a small focus in which a current is supplied to heat only a region narrower than the entire region of the electron emission surface 101 (hereinafter referred to as “half lighting”) to emit electrons.
Specifically, when the entire region (see a hatched area with positive slope in the figure) of the electron emission surface 101 is to be heated as illustrated in FIG. 8A, a current is supplied from the full-lighting current-supply heating legs 102 and 103 (see the hatched area with positive slope in the figure) to heat the entire surface. On the other hand, when the electron emission surface is locally lighted to restrict the electron emission range for reducing a focal spot, a current is supplied from the half-lighting current-supply heating legs 104 and 105 (see the hatched area with positive slope in the figure) to light and heat only the hatched area with positive slope in the figure as illustrated in FIG. 8B. In the full lighting, the current supply path becomes the full-lighting current-supply heating leg 102→the base part of the full-lighting current-supply heating leg 102→the base part of the half-lighting current-supply heating leg 105→the base part of the half-lighting current-supply heating leg 104→the base part of the full-lighting current-supply heating leg 103→the full-lighting current-supply heating leg 103. In the half lighting, the current supply path becomes the half-lighting current-supply heating leg 104→the base part of the half-lighting current-supply heating leg 104→the base part of the half-lighting current-supply heating leg 105→the half-lighting current-supply heating leg 105. In this way, the current supply path is changed to switch the electron emission region.
When an emitter is used as an X-ray electron source, a focusing electrode C is disposed on the front surface (emission surface) of the emitter E for focusing thermoelectrons B on a target T composed of an anode as illustrated in FIG. 9. If the flat emitter including the legs for the current supply and heating has a plate-like shape as illustrated in FIG. 7 or 8, there arise problems such that (1) a complicated mechanism is required to dispose the emitter relative to the electrode, and further, the work for disposing the emitter becomes complicated, and (2) the electron emission region (see the thermoelectrons B in FIG. 9) spreads to the legs upon an irradiation of high current, which fails to form a desired focal spot shape.
In view of this, in the conventional flat emitter illustrated in FIG. 7, the current-supply heating legs 102 and 103 are folded from their base parts at 90 degrees as illustrated in FIG. 10 so as to allow only the necessary electron emission region to face the electrode. The legs 102 and 103 are folded at 90 degrees at portions indicated by a broken line in FIG. 7 to place them to be opposite to each other as illustrated in FIG. 10. FIG. 10A is a schematic perspective view of the conventional flat emitter, FIG. 10B is a schematic front view viewed from front F in FIG. 10A, FIG. 10C is a schematic side view viewed from a side face S in FIG. 10A, and FIG. 10D is a schematic plan view viewed from a top surface U in FIG. 10A.
When the emitter is assembled to an X-ray tube, the emitter is generally assembled as illustrated in FIG. 11 for simplifying an assembling process to the X-ray tube. Specifically, terminals of the current-supply heating legs 102 and 103 are fixed to electrode bars ER (an insulating member such as a ceramic and an electrode bar are collectively referred to as a “base” below) brazed to an insulating member I such as a ceramic with brazing or welding, and then, the legs 102 and 103 are assembled to an X-ray tube.
For the reason described above, in the conventional flat double emitter illustrated in FIG. 8, four current-supply heating legs 102 to 105 are folded from their base parts at 90 degrees as illustrated in FIG. 12, as in the conventional flat emitter. The legs 102 to 105 are folded at portions indicated by a broken line in FIG. 8 at 90 degrees, and the four legs 102 to 105 are placed such that the full-lighting current-supply heating legs 102 and 103 are opposite to each other as symmetric about a center axis, which is a vertical axis of the electron emission surface 101 and passes through the center of the electron emission surface 101, and the half-lighting current-supply heating legs 104 and 105 are opposite to each other as symmetric about the center axis, as illustrated in FIG. 12. The flat double emitter in which the legs 102 to 105 are linearly formed as illustrated in FIG. 12 is referred to as a “double emitter type 1” below. FIG. 12A is a schematic perspective view of the double emitter type 1, FIG. 12B is a schematic front view viewed from front F in FIG. 12A, FIG. 12C is a schematic side view viewed from a side face S in FIG. 12A, and FIG. 12D is a schematic plan view viewed from a top surface U in FIG. 12A.
In order to assemble the double emitter type 1 to an X-ray tube, terminals of the four current-supply heating legs 102 to 105 are fixed to the electrode bars ER (base) brazed to the insulating member I with welding as illustrated in FIG. 13. FIG. 13A is a schematic perspective view illustrating that the double emitter type 1 is assembled to the base for the X-ray tube, and FIG. 13B is a schematic plan view viewed from the bottom surface in FIG. 13A.
However, the structure of the double emitter type 1 has a problem of very small space between the electrode bars ER of the base as apparent from FIG. 13. Therefore, it is extremely difficult to fix the electrode bars ER to the insulating member such as a ceramic while providing electrical insulation between the electrode bars ER. It is considered that the electrode bar ER is made thin. However, this is not preferable from the viewpoint of strength.
In view of this, the structure described below is considered for surely providing insulation between the electrode bars ER. Specifically, as illustrated in FIG. 14, the four current-supply heating legs 102 to 105 are folded from their base parts at 90 degrees, and all current-supply heating legs 102 to 105 are bent plural times (twice in FIG. 14) in zigzag to widen the terminals of the legs 102 to 105. The flat double emitter in which all legs 102 to 105 are bent plural times in zigzag as illustrated in FIG. 14 is referred to as a “double emitter type 2” below. FIG. 14A is a schematic perspective view of the double emitter type 2, FIG. 14B is a schematic front view viewed from front F in FIG. 14A, FIG. 14C is a schematic side view viewed from a side face S in FIG. 14A, and FIG. 14D is a schematic plan view viewed from a top surface U in FIG. 14A.
In FIG. 14, the legs 102 to 105 are bent at right angle (90 degrees), and then, further bent at right angle (90 degrees) in zigzag. Thus, the legs 102 to 105 are bent twice in zigzag. The number of bending the legs in zigzag is not limited to two in FIG. 14. The number may be two or more. As described above, the legs 102 to 105 are bent plural times in zigzag, whereby the directions in which the legs 102 to 105 extend at their terminals can be aligned parallel to one another.
In order to assemble the double emitter type 2 to an X-ray tube, terminals of the four current-supply heating legs 102 to 105 are fixed to the electrode bars ER (base) brazed to the insulating member I with welding as illustrated in FIG. 15. In the structure of the double emitter type 2, the space between the electrode bars ER of the base is large as apparent from FIG. 15, which makes it easy to fix the electrode bars ER while providing electrical insulation between the electrode bars ER.
However, in the double emitter type 2, all legs are long, and have a zigzag shape. Therefore, keeping balance is difficult, and the legs are difficult to be fixed to the base with high precision. Accordingly, there arise the problems described below.
(1) The electron emission surface is deformed due to unnatural stress caused by the fixation, which fails to form a desired focal spot shape.
(2) The deformation in the above (1) causes positional deviation in the double emitter type 2, which makes it difficult to dispose the double emitter type 2 relative to the focusing electrode C (see FIG. 9) with high precision. If the double emitter type 2 cannot be disposed with high precision, the focal spot might be enlarged or asymmetry might occur, entailing a problem in which a desired focal spot shape cannot be obtained.
The present invention is accomplished in view of the above circumstances, and aims to provide a flat emitter that is easily assembled to an electron source for an X-ray tube or an electron source for another use and has a shape capable of attaining high positional precision.